Download Physics 127 Descriptive Astronomy Homework #6 Key (Chapter 3

Physics 127 Descriptive Astronomy
Homework #6 Key (Chapter 3)
Winter 2015
3-3. Give a word or phrase that corresponds to the following standard abbreviations:
a) km - kilometer
b) cm - centimeter
c) s - second
d) km/s - kilometers per second
e) mi/h - miles per hour f) m - meters
g) m/s - meters per second h) h - hour
i) ly - light year
j) g - gram
k) kg – kilogram
3-7. In what direction does a planet move relative to the stars when it is in direct motion? When it is in
retrograde motion? How do these compare with the direction in which we see the Sun move
relative to the stars?
In direct motion a planet moves eastward with respect to the background stars. In retrograde motion is relative motion is
westward. The sun moves eastward with respect to the background stars.
3-12. Which planets can never be seen at opposition? Which planets can never be seen at inferior
conjunction? Explain your answers.
The inferior planets, Mercury and Venus, are never seen at opposition because that would require that their direction in
the sky be opposite that of the sun. Since their orbits lie inside the earth’s orbit, that would be impossible. The superior
planets, Mars, Jupiter, Saturn, Uranus and Neptune are never seen at inferior conjunction because that would require
that they be between us and the sun. Since their orbits lie outside the earth’s orbit, that would be impossible.
3-13. What is the difference between the synodic period and the sidereal period of a planet?
The synodic period is the time required for a planet to move from a particular planetary aspect (inferior conjunction,
greatest eastern or western elongation, or superior conjunction for an inferior planet; opposition, eastern or western
quadrature or conjunction for a superior planet) to the next repetition of that particular aspect. More simply stated, a
planet’s synodic period is the time required for the earth to gain exactly one orbital lap on a superior planet or for an
inferior planet to gain exactly one orbital lap on the earth. Its sidereal period is the time required for any planet to
complete exactly one revolution about the sun.